This invention relates to a hydrogen storage apparatus that supplies hydrogen to a fuel cell or other hydrogen-using devices, and to a method of charging hydrogen to the hydrogen storage apparatus.
On environmental considerations in recent years, attention is being given to a fuel cell electric vehicle (FCEV) in which carbon dioxide emissions causing global warming may be reduced. The fuel cell electric vehicle includes a fuel cell (FC) that triggers an electrochemical reaction of hydrogen (H2) with oxygen (O2) in the air, and electric power produced by the fuel cell is supplied to a traction motor to generate driving force.
Among apparatuses for supplying hydrogen to the fuel cell or other hydrogen-using devices, a hydrogen storage apparatus using a gastight pressure vessel containing a hydrogen occlusive alloy is very popular, which gastight pressure vessel will hereinafter be referred to as xe2x80x9cMH tankxe2x80x9d. The MH is an abbreviation for metal hydride. The hydrogen occlusive alloy is an alloy that may absorb (or occlude) a great mass of hydrogen (or hydrogen gas), and give out (or emit) the occluded hydrogen as necessary. The hydrogen occlusive alloy characteristically generates heat upon occlusion of hydrogen, raising a temperature thereof, and absorbs heat upon emission of hydrogen, lowering the temperature thereof. In addition, as shown in FIG. 6, an equilibrium hydrogen pressure in the hydrogen occlusive alloy characteristically increases with a rise in temperature of the hydrogen occlusive alloy (the phenomenon is called high-temperature shift), and lowers with a drop in temperature of the hydrogen occlusive alloy (the phenomenon is called low-temperature shift). To be more specific, upon occlusion, a rise in temperature of the hydrogen occlusive alloy would raise the equilibrium hydrogen pressure, and thus reduce the amount of hydrogen that can be occluded. On the other hand, upon emission, a drop in temperature of the hydrogen occlusive alloy would lower the equilibrium hydrogen pressure, and thus inhibit hydrogen occluded from being emitted sufficiently.
Accordingly, the hydrogen storage apparatus is configured to cool the MH tank upon occlusion of hydrogen to prevent a rise in temperature, and to heat the MH tank upon emission of hydrogen to prevent a drop in temperature.
However, for example, upon startup of the hydrogen-using device (esp. upon cold startup thereof) or on other occasions where no heat source may be available for heating the MH tank, hydrogen cannot be sufficiently emitted from the MH tank.
Therefore, the hydrogen storage apparatus usually incorporates an auxiliary tank capable of emitting hydrogen at a low temperature, or is otherwise configured to make up a shortfall.
For instance, Japanese Laid-Open Patent Application, Publication No. 5-106513 discloses a hydrogen supplying apparatus for a hydrogen engine including a normal-time hydrogen occlusive tank and a startup-time hydrogen occlusive tank. The startup-time hydrogen occlusive tank in the hydrogen supplying apparatus includes a hydrogen occlusive alloy capable of emitting hydrogen under low-temperature conditions without application of heat. Since an internal pressure of the startup-time hydrogen occlusive tank tends to build up, the startup-time hydrogen occlusive tank is made relatively compact to acquire resistance to pressure while restricting increase in weight. Upon startup under cold conditions, the hydrogen supplying apparatus supplies hydrogen from the startup-time hydrogen occlusive tank to start a hydrogen engine.
Japanese Laid-Open Patent Application. Publication No. 7-101255 discloses a hydrogen automobile including a main fuel tank filled with a hydrogen occlusive alloy, and a small-volume subordinate fuel tank. In this hydrogen automobile, hydrogen is supplied from the subordinate fuel tank to an engine thereof upon startup. The subordinate fuel tank may be charged with hydrogen from the main fuel tank on some occasions. The subordinate fuel tank may be filled with a hydrogen occlusive alloy in some instances.
Japanese Laid-Open Patent Application, Publication No. 9-142803 discloses a hydrogen supplying apparatus in which a gas tank is coupled with a gas vent of a hydrogen occlusive alloy tank filled with a hydrogen occlusive alloy via a check valve, and a hydrogen feed pipe for feeding hydrogen to an external device (i.e., fuel cell) is coupled with a gas exhaust vent of the gas tank. This hydrogen supplying apparatus has the gas tank provided between the hydrogen occlusive alloy tank and the fuel cell, and is configured to supply hydrogen from the gas tank to the fuel cell upon startup by utilizing sufficiently high pressure (approximately 0.9 MPa) kept by the check valve in the gas tank. On the contrary, when the temperature of gas emitted from the fuel cell itself rises after startup, the internal pressure of the hydrogen occlusive alloy tank builds up, so that hydrogen is supplied from the hydrogen occlusive alloy tank by the action of the check valve.
Japanese Laid-Open Patent Application, Publication No. 2000-12062 discloses a hydrogen supplying apparatus including a main hydrogen storage tank accommodating a higher-temperature hydrogen occlusive alloy that may emit hydrogen under conditions of predetermined higher temperature, and a subordinate hydrogen storage tank accommodating a lower-temperature hydrogen occlusive alloy that may emit hydrogen under conditions of lower temperature below the predetermined higher temperature, and the like. This hydrogen supplying apparatus supplies hydrogen from the subordinate hydrogen storage tank to an external load (i.e. fuel cell) upon startup when the internal pressure of the main hydrogen storage tank is low, and starts supplying hydrogen from the main hydrogen storage tank to the external load some time after startup when the internal pressure of the main hydrogen storage tank gets higher.
However, the above-cited prior arts, which might possibly clear up difficulties in supplying hydrogen during startup, could not overcome disadvantages in weight. To be more specific, the MH tank accommodating (or filled with) a hydrogen occlusive alloy is compact in size, but heavy in weight, and thus would disadvantageously entail poor fuel efficiency when installed for example in a fuel cell electric vehicle. In addition, the heavy weight would disadvantageously make it difficult to handle the apparatus. Accordingly, weight reduction is critical. On the other hand, reduced amount of storable hydrogen that might take place due to the weight reduction would yield unfavorable results, eg., shorten a distance the fuel cell electrical vehicle may travel.
Moreover, heat generated upon occlusion of hydrogen in the MH tank would slow down a hydrogen charging speed (occlusion speed) of the hydrogen storage apparatus. This would disadvantageously result in longer hydrogen charging time.
Therefore, it is an object of the present invention to provide a hydrogen storage apparatus and hydrogen charging method in which the above-described disadvantages may be eliminated, and more specifically to provide a hydrogen storage apparatus fit to install in a vehicle.
In order to achieve the above object, the present inventors have discovered as a result of their thorough study that a hybrid system formed by combining a hydrogen storage means accommodating a hydrogen occlusive alloy, and a hydrogen tank containing hydrogen in gaseous form, each of which is configured to have an internal pressure set appropriately, may work with the above disadvantages surmounted, and consequently have brought the invention to perfection.
A method of charging hydrogen to a hydrogen storage apparatus (in claim 1) as one exemplified aspect of the present invention, in which the above-described disadvantages are eliminated, comprises the steps of filling hydrogen to a hydrogen storage means provided in the hydrogen storage apparatus to accommodate a hydrogen occlusive alloy, and filling hydrogen to a hydrogen tank provided in the hydrogen storage apparatus separately from the hydrogen storage means to store hydrogen in gaseous form, and hydrogen is filled so that a pressure in the hydrogen tank is kept higher than that in the hydrogen storage means.
An MH tank as will be explained later under the heading xe2x80x9cDetailed description of the preferred embodimentsxe2x80x9d corresponds to the hydrogen storage means, in which a hydrogen occlusive alloy is stored. On the other hand, a high-pressure hydrogen tank as will be explained later under the same heading corresponds to the hydrogen tank, in which hydrogen is stored under high pressure. Of these two elements, the hydrogen storage means has an optimal pressure for storing hydrogen (storage pressure), beyond which increase in the amount of hydrogen storable would not be commensurate with the increase in pressure, by reason of equilibrium hydrogen pressure of the hydrogen occlusive alloy, and a structural capacity to resist pressure of the hydrogen storage means. Conversely, securing the capacity to resist pressure would require a thick material to constitute the hydrogen storage means, and would increase the weight of the hydrogen storage means. In contrast, the hydrogen tank, of which internal conditions are typically governed by the Boyle-Charles law, may accommodate more hydrogen as the pressure builds up more and more. Accordingly, this method of charging hydrogen to the hydrogen storage apparatus is so configured that the hydrogen tank stores hydrogen under higher pressure than the hydrogen storage means does.
In addition, occlusion of hydrogen in the hydrogen occlusive alloy generates great heat; however, according to this method, hydrogen is stored in the hydrogen tank as well as the hydrogen storage means containing an intense heat source. Therefore, unlike a case where hydrogen is stored only in the hydrogen storage means, this method serves to reduce the load on the hydrogen storage means, thus allowing the amount of heat generated in the hydrogen storage alloy to decrease. Although heat is generated due to adiabatic compression of hydrogen or the like upon charging hydrogen into the hydrogen tank as well, the amount of heat generated is less than that generated in the hydrogen storage means accommodating the hydrogen occlusive alloy. Consequently, the amount of heat generated from the hydrogen occlusive alloy may be restricted, and thus the time required to fill the hydrogen storage means with hydrogen may be saved.
According to another aspect of the present invention, there is provided a hydrogen storage apparatus that stores hydrogen to be supplied to a hydrogen-using device (in claim 2). This hydrogen storage apparatus comprises a hydrogen storage means that accommodates a hydrogen occlusive alloy and may supply hydrogen to the hydrogen-using device, a hydrogen tank that stores hydrogen in gaseous form and may supply the hydrogen to the hydrogen-using device, a first hydrogen filling line to fill hydrogen to the hydrogen storage means, a second hydrogen filling line to fill hydrogen to the hydrogen tank, and a decompressing means provided in the first hydrogen filling line to reduce a pressure of hydrogen fed through the first hydrogen filling line to maintain the pressure in a predetermined pressure range.
The hydrogen storage apparatus supplies hydrogen stored therein to a fuel cell or other type of hydrogen-using device, and becomes empty. Then, hydrogen is charged into the hydrogen storage apparatus. In this construction, the pressure of hydrogen filled from the first hydrogen filling line into the hydrogen storage means in the hydrogen storage apparatus is reduced to a predetermined range of pressure using the decompressing means (e.g., a regulator, a throttle, a pressure-reducing valve, etc.).
The decompression is carried out so that a pressure in the hydrogen tank is higher than that in the hydrogen storage means. Preferably, the predetermined pressure range may be set around a critical level that makes the hydrogen storage means unable to substantially increase amounts of occlusive hydrogen any more.
The hydrogen storage means may comprise a plurality of hydrogen storage tanks each accommodating the hydrogen occlusive alloy. The hydrogen storage means accommodating a hydrogen occlusive alloy is cooled upon occlusion of hydrogen, and heated upon emission of hydrogen. Accordingly, if a surface area of the hydrogen storage means is made broader by providing a plurality of hydrogen storage tanks, then heating/cooling operations may become more efficient.
The first and second hydrogen filling lines may include a common hydrogen filling port through which hydrogen is filled. Alternatively, the first and second hydrogen filling lines may each include a separate and differently shaped hydrogen filling port through which hydrogen is filled.
A hydrogen storage apparatus as yet another aspect of the present invention (in claim 7) also comprises a hydrogen storage means that accommodates a hydrogen occlusive alloy and may supply hydrogen to the hydrogen-using device, and a hydrogen tank that stores hydrogen in gaseous form and may supply the hydrogen to the hydrogen-using device, in which a pressure of hydrogen filled to the hydrogen storage means is maintained in a predetermined pressure range approximate to a critical level that makes the hydrogen storage means unable to substantially increase amounts of occlusive hydrogen any more, and the hydrogen storage apparatus has optimal volume and weight.
This construction allows the hydrogen storage apparatus to exert the same operation as above, while achieving a reduction in weight.
Other objects and further features of the present invention will become readily apparent from the following description of preferred embodiments with reference to accompanying drawings.